School meals should include fewer puddings and more fresh vegetables, according to a report.
Obesity Action Scotland (OAS) said improvements to school meals could play an important part in reducing childhood obesity.
It wants to highlight the issue ahead of the council elections in two weeks' time.
The Scottish government said a review of school food and drink nutritional standards was under way.
OAS is calling on local government election candidates to commit to transform school meals in Scotland "from a feeding culture to an eating culture".
The organisation said it wanted unprocessed or "minimally processed" foods used wherever possible and vegetables, soup and salads prioritised over puddings.
Its report found school dinners varied "dramatically" across Scotland, with many primary schools serving puddings more often than soup.
Two-thirds of primary school pupils in Scotland eat school meals and OAS said this provided a "unique opportunity" to drive dietary change.
Lorraine Tulloch, from OAS, said: "We are calling on local government election candidates to commit to transform school meals across Scotland to ensure children have a healthy and happy experience with food.
"Change is possible and we have highlighted areas where that change is starting to happen, but more action is needed and greater priority and attention needs to be given to this subject to ensure we offer all our children the best start in life."
The Scottish government said all local authorities had a duty to provide school meals that meet strict nutritional requirements.
A spokesman added: "Healthy Eating In Schools guidance exists to help local authorities and schools meet the current requirement. This includes advice on making puddings healthier.
"The deputy first minister confirmed on 5 March 2017 that a review of these school food and drink nutritional standards is under way to ensure the nutritional standards are the best they can be.
"The review will also consider whether school food provision can be further improved, in light of the latest evidence from the scientific advisory committee on nutrition."
A study last year found that since 2005 more than one in 10 children in Scotland had started primary school overweight or obese.
The figures showed almost 83,000 four to five-year-olds entered P1 carrying excess weight between 2005-2006 and 2014-2015.